An NMR Method To Identify Nondestructively Chemical Compounds Bound to a Single Solid-Phase-Synthesis Bead for Combinatorial Chemistry Applications

High-resolution magic angle spinning (1) H NMR measurement of ligand concentration in solvent-saturated chromatographic beads

A method based on (1) H high-resolution magic angle spinning NMR has been developed for measuring concentration accurately in heterogeneous materials like that of ligands in chromatography media. Ligand concentration is obtained by relating the peak integrals for a butyl ligand in the spectrum of a water-saturated chromatography medium to the integral of the added internal reference. The method is fast, with capacity of 10 min total sample preparation and analysis time per sample; precise, with a reproducibility expressed as 1.7% relative standard deviation; and accurate, as indicated by the excellent agreement of derived concentration with that obtained previously by (13) C single-pulse excitation MAS NMR. The effects of radiofrequency field inhomogeneity, spin rate, temperature increase due to spinning, and distribution and re-distribution of medium and reference solvent both inside the rotor during spinning and between bulk solvent and pore space are discussed in detail. © 2016 The Authors Magnetic Resonance in Chemistry published by John Wiley & Sons Ltd.

Revealing protein structures in solid-phase peptide synthesis by 13C solid-state NMR: evidence of excessive misfolding for Alzheimer's β

Solid-phase peptide synthesis (SPPS) is a widely used technique in biology and chemistry. However, the synthesis yield in SPPS often drops drastically for longer amino acid sequences, presumably because of the occurrence of incomplete coupling reactions. The underlying cause for this problem is hypothesized to be a sequence-dependent propensity to form secondary structures through protein aggregation. However, few methods are available to study the site-specific structure of proteins or long peptides that are anchored to the solid support used in SPPS. This study presents a novel solid-state NMR (SSNMR) approach to examine protein structure in the course of SPPS. As a useful benchmark, we describe the site-specific SSNMR structural characterization of the 40-residue Alzheimer's β-amyloid (Aβ) peptide during SPPS. Our 2D (13)C/(13)C correlation SSNMR data on Aβ(1-40) bound to a resin support demonstrated that Aβ underwent excessive misfolding into a highly ordered β-strand structure across the entire amino acid sequence during SPPS. This approach is likely to be applicable to a wide range of peptides/proteins bound to the solid support that are synthesized through SPPS.

Application of HR-MAS NMR in the solid-phase synthesis of a glycopeptide using Sieber amide resin

The solid-phase synthesis (SPS) of a structurally complex glycopeptide, using Sieber amide resin, was monitored by high resolution magic angle spinning NMR, demonstrating the further application of this technique. A synthetic peptidoglycan derivative, a precursor of a biologically active PGN, known to be involved in the cellular recognition, was prepared by SPS. The synthesis involved the preparation of an N-alloc glucosamine moiety and the synthesis of a simple amino acid sequence L-Ala-D-Glu-L-Lys-D-Ala-D-Ala. Last step consisted the coupling, on solid-phase, of the protected muramyl unit to the peptide chain. Proton spectra with good suppression of the polystyrene signals in swollen resin samples were obtained in DMF-d(7) as a solvent and by using a nonselective 1D TOCSY/DIPSI-2 scheme, thus allowing to follow the SPS without losses of compound and cleavage from the resin. The assignment of the proton spectra of the resin-bound amino acid sequence and of the bound glycopeptide was achieved through the combination of MAS COSY, TOCSY and NOESY.

HRMAS 1H NMR conformational study of the resin-bound amyloid-forming peptide GNNQQNY from the yeast prion Sup35

The conversion of soluble proteins to insoluble amyloid fibrils is associated with numerous human diseases. The peptide GNNQQNY is a short segment of the yeast prion protein Sup35 that previously has been found to form amyloid fibrils in a similar manner to the protein itself. The approach taken in this work was to attach this peptide sequence to an insoluble polymer matrix through solid phase peptide synthesis and give it the internal freedom to fold into its local conformation in an organic solvent. Observation of its monomeric structure, free from the effects of aggregation, entropic solvent effects, and neighboring molecules, was possible by two-dimensional high-resolution magic angle spinning (1)H NMR spectroscopy. Analysis of the through-bond correlations and through-space interactions observed in the spectra, combined with global energy minimization via computational studies, led to the observation that the peptide chain adopts a compact beta-like turn at the central hydrophilic residues. The technique of peptide attachment to a polymer resin and observation by NMR may allow for future study of single peptide fragments prone to aggregation.

Inverse H-C ex situ HRMAS NMR experiments for solid-phase peptide synthesis

The growing importance of solid-phase peptide synthesis (SPPS) has necessitated the development of spectroscopic experiments that can be used to obtain structural and conformational information on resin-bound peptides. Despite the utility of two-dimensional high-resolution magic angle spinning (HRMAS) NMR experiments that provide homonuclear shift correlations, experiments that provide heteronuclear shift correlations are necessary for complex conformational and structural elucidatory problems. Here we report the optimization and implementation of non-gradient inverse NMR experiments for acquiring the 1H-13C shift correlations of resin-bound peptides. The use of non-gradient experiments is advantageous as many magic angle spinning (MAS) probes do not possess gradient coils. An HRMAS BIRD-HMQC experiment with a reduced 1JCH constant has proven very suitable for obtaining one-bond correlations. Long-range correlations can be interpolated by using a non-gradient HRMAS CT-HMBC-1 experiment where the resulting data is processed with forward linear prediction. It has been shown that removing the effects of 1H-1H J-modulation is crucial in order to view cross peaks that correspond to long-range correlations. Additionally, both experiments prove extremely useful over routine one-dimensional 13C HRMAS experiments for extracting carbon chemical shift data. The non-gradient HRMAS BIRD-HMQC and CT-HMBC-1 experiments can be used to assist in conformational analysis and to identify and deconvolute situations where accidental equivalence and seemingly correlated isochronous signals arise.

Classification of spectroscopically encoded resins by Raman mapping and infrared hyperspectral imaging

Barcoded resins (BCRs) were recently introduced as a potential platform for pre-encoded multiplexed synthesis, screening, and biomedical diagnostics. A key step toward the development of this strategy is the ability to rapidly interrogate and classify the BCRs in a high-throughput, noninvasive manner. Here, we describe a one-step strategy based on Raman mapping and Fourier transform infrared imaging to classify and spatially resolve randomly distributed BCRs. To illustrate this methodology, mixtures of up to 25 different BCRs were imaged and classified with 100% confidence. This strategy can be readily extended to a larger pool of resins, provided each BCR features a unique vibrational fingerprint (spectroscopic barcode). We have also established that reliable single-bead Raman spectra can be recorded in 10 ms, thus confirming that Raman mapping, in particular, could be a very fast method to classify the BCRs.

Spectroscopically encoded resins for high throughput imaging time-of-flight secondary ion mass spectrometry

Spectroscopic barcoding was recently introduced as a new pre-encoding strategy wherein the resin beads are not just carriers for solid phase synthesis, but are, in addition, the repository of the synthetic scheme to which they were subjected. To expand the repertoire of spectroscopically barcoded resins (BCRs), here we introduce a new family of halogenated polystyrene-based polymers designed for high-throughput combinatorial analysis using not only infrared and Raman spectroscopy but also imaging time-of-flight secondary ion mass spectrometry (ToF-SIMS). In particular, we have established that (a) the halogen content of these new resins can be used as an encoding element in quantitative imaging ToF-SIMS and (b) the number of styrene monomers used to generate unique vibrational fingerprints can be significantly reduced by using monomers in different molar ratios. The combination of quantitative imaging ToF-SIMS and vibrational spectroscopy is anticipated to dramatically increase the repertoire of possible BCRs from a few hundreds to several thousands.

MALDI-TOF MS analysis of soluble PEG based multi-step synthetic reaction mixtures with automated detection of reaction failure

Macromolecules of tunable solubility, used to mimic inert insoluble materials while maintaining solution conditions, allowed the performance of efficient supported organic chemistry and facilitated in situ reaction monitoring. To satisfy the high throughput requirements of automated synthetic processes, organic syntheses carried out on bifunctional polyethylene glycol polymers (PEG(3400)-OH) were monitored step-by-step by matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). A protocol was designed to control the ionization mechanism of such polymers exhibiting high affinity for alkali metal cations. Automated, rapid, and reliable data interpretation was performed by an in-house developed visual basic application relying on the sodiated ion accurate monoisotopic mass measurement. The methodology was illustrated through the monitoring of a six-step synthetic scheme.

Characterisation of glycoprotein ligands synthesised using solid-phase combinatorial chemistry

A combination of rational design based on mimicking natural protein-carbohydrate interactions and solid-phase combinatorial chemistry has led to the identification of an affinity ligand which displays selectivity for the mannose moiety of glycoproteins. The ligand was initially identified as 32/18, a triazine scaffold substituted with 2-acetylpyrrole (32) and 5-aminoindan (18). However, characterisation of the immobilised ligand by release from the matrix via a cleavable linker, (4s,5s)-4,5-di(aminomethyl)-2,2-dimethyldioxolane, and using a non-destructive on-resin method, 13C NMR spectroscopy, confirmed that the putative ligand 32/18 was, in fact, 18/18, the disubstituted 5-aminoindan. 1H NMR studies on the interaction of alpha-D-methylmannoside with the ligand 18/18 in solution confirm the involvement of the hydroxyl group in the C-2 position.

Probing Molecular Dynamics in Chromatographic Systems Using High-Resolution 1H Magic-Angle-Spinning NMR Spectroscopy: Interaction between p-Xylene and C18-Bonded Silica

The exact nature of the interaction between small molecules and chromatographic solid phases has been the subject of much research, but detailed understanding of the molecular dynamics in such systems remains elusive. High-resolution (1)H magic-angle-spinning (MAS) NMR spectroscopy has been applied to the investigation of C18-bonded silica material as used in chromatographic separation techniques together with an adsorbed model analyte, p-xylene. Two distinct p-xylene and water environments were identified within the C18-bonded silica through the measurement of (1)H NMR chemical shifts, T(1) and T(2) relaxation times and diffusion coefficients, including their temperature dependence. The results have been analyzed in terms of two environments, p-xylene within the C18 chains, in slow exchange on the NMR time scale with p-xylene in a more mobile state adsorbed as a layer in close proximity to the C18 particles, but which is distinct from free liquid p-xylene. The techniques used here could have more general applications, including the study of drug molecules bound into phospholipid membranes in micelles or vesicles.

Solid-Phase Synthesis of O-Glycosylated Nα-Fmoc Amino Acids and Analysis by High-Resolution Magic Angle Spinning NMR

Direct O-glycosylation of amino acids bound to TentaGel resin with a number of glycosyl trichloroacetimidate donors results in high yields. The glycosylation reaction can be easily monitored by analyzing the bead-bound amino acids with high-resolution magic angle spinning (HR-MAS) NMR. These studies pave a new way for the construction of "one-bead one-compound" O-glycopeptide libraries with standard amino acid building blocks and appropriate glycosyl trichloroacetimidate donors.

Preparation, physical properties, on-bead binding assay and spectroscopic reliability of 25 barcoded polystyrene-poly(ethylene glycol) graft copolymers

Here we describe the preparation of 25 beaded polystyrene-poly(ethylene glycol) graft copolymers from six spectroscopically active styrene monomers: styrene, 2,5-dimethylstyrene, 4-methylstyrene, 2,4-dimethylstyrene, 4-tert-butylstyrene, and 3-methylstyrene. These polymers were thoroughly characterized by Raman, infrared, and (1)H/(13)C NMR spectroscopies, and differential scanning calorimetry. Determination of the swelling properties, peptide synthesis, and on-bead streptavidin-alkaline phosphatase (SAP) binding assay further established that their physical and chemical properties where not significantly altered by the diversity of their encoded polystyrene core. Each of the 25 resins displayed a unique Raman and infrared vibrational fingerprint, which was converted into a "spectroscopic barcode". The position of each bar matches the peak wavenumber in the corresponding spectrum but is independent of its intensity. From this simplified representation similarity maps comparing 35 000 resin pairs were generated to establish the spectroscopic barcoding as a reliable encoding methodology. In effect, in 99% of the cases, the highest similarity coefficients were obtained for resin pairs prepared from the same styrene derivatives even after SAP binding assay. We have also shown that a small but unique combination of a resin's vibrations (30-40%) is sufficient for its identification. However, in rare cases where a resin's vibrational signature has been severely compromised, both the Raman and infrared barcodes were synergistically and reliably utilized to unequivocally identify its chemical make up.

Method for analysis of polymer-supported organic compounds using mass spectrometry direct insertion

A new approach on the use of mass spectrometry direct-insertion and a quadrupole detector for analysis of organic compounds supported in solid phase has been developed. This is a simple and efficient method based on cleavage due to the thermal-instability of the benzylic group of most commercial resins. The cleavage of supported compounds takes place in the spectrometer as a consequence of the high temperature in the instrument's chamber. These compounds are detected using a similar fragmentation pattern and a molecular ion corresponding to the same compound obtained by traditional synthesis. Polymer degradation fragments do not interfere with the spectrum interpretation, because only a few peaks and low intensities are detected. We report here the identification of different types of compounds supported in Merrifield resin, such as bis-o-aminobenzamides and simple aromatic and aliphatic compounds, using this new approach.

Production, analysis, and application of spatially resolved shells in solid-phase polymer spheres

Two-photon fluorescence microscopy has been used to interrogate the interior functionality of polymer resin beads. By employing this technique, the spatial distribution of the initial functionality contained within the polymer matrix has been determined. Spatially resolved, concentric shells were then produced synthetically in these polymer spheres via a series of protection/deprotection reactions in which two-photon fluorescence microscopy was employed to monitor each successive step. To demonstrate the potential utility of these techniques in combinatorial screening, a set of beads was prepared containing a unique tripeptide sequence in each of the three concentric shells within each individual bead. The set was then screened for the binding affinity of each tripeptide toward a fluorescent ligand.

Solid-phase enolate chemistry investigated using HR-MAS NMR spectroscopy

Supported P4-t-Bu enolate chemistry of phenylacetyloxymethyl polystyrene (PS) resin was investigated using high-resolution magic angle spinning (HR-MAS) NMR spectroscopy. Direct analysis of the crude reaction suspensions through the use of a diffusion filter (DF) allowed a rapid selection of the optimal experimental conditions, but also the characterization of the enolate on the solid phase. Comparison with solution experiments and literature data allowed us to address partially the structure of the enolate. HR-MAS NMR spectra of the enolate revealed also a tight interaction of P4-t-Bu base with the polymer matrix.

A one-bead, one-stock solution approach to chemical genetics: part 1

BACKGROUND

In chemical genetics, small molecules instead of genetic mutations are used to modulate the functions of proteins rapidly and conditionally, thereby allowing many biological processes to be explored. This approach requires the identification of compounds that regulate pathways and bind to proteins with high specificity. Structurally complex and diverse small molecules can be prepared using diversity-oriented synthesis, and the split-pool strategy allows their spatial segregation on individual polymer beads, but typically in quantities that limit their usefulness.

RESULTS

We report full details of the first phase of our platform development, including the synthesis of a high-capacity solid-phase bead/linker system, the development of a reliable library encoding strategy, and the design of compound decoding methods both from macrobeads and stock solutions. This phase was validated by the analysis of an enantioselective, diversity-oriented synthesis resulting in an encoded 4320-member library of structurally complex dihydropyrancarboxamides.

CONCLUSIONS

An efficient and accessible approach to split-pool, diversity-oriented synthesis using high-capacity macrobeads as individual microreactors has been developed. Each macrobead contains sufficient compound to generate a stock solution amenable to many biological assays, and reliable library encoding allows for rapid compound structure elucidation post-synthesis. This 'one-bead, one-stock solution' strategy is a central element of a technology platform aimed at advancing chemical genetics.

1H NMR characterization of the product from single solid-phase resin beads using capillary NMR flow probes

A capillary NMR flow probe was designed to generate high-resolution (1)H NMR spectra at 600 MHz from the cleaved product of individual 160-microm Tentagel combinatorial chemistry beads. By injecting a dissolved sample sandwiched between an immiscible, perfluorinated organic liquid directly into the probe, NMR spectra of the product cleaved from single beads were acquired in just 1 h of spectrometer time without diffusional dilution. Sample handling efficiency on the single bead scale was comparable to that obtained with a bulk sample. Using the relative intensity of the DMSO-d(5)H versus the analyte signals in a fully relaxed CPMG spectrum, the amount of product cleaved from a single bead was determined to be 540+/-170 pmol in one of the samples. Following the NMR data collection, the samples were examined with electrospray ionization mass spectrometry to provide additional structural information. By coupling with microliter-volume fluidic capabilities, the capillary flow probe described here will enable multidimensional characterization of single solid-phase resin products in an online manner.

Development and application of a carbonyl-(13)C-enriched backbone amide linker for solid-phase reaction monitoring

The synthesis and application of a carbonyl-(13)C backbone amide linker are described. The labeled unit is conveniently mixed with commercial resins, providing a rapid means of monitoring chemistry performed with this linker on solid support using conventional (13)C NMR methods.

Evidence of secondary structure by high-resolution magic angle spinning NMR spectroscopy of a bioactive peptide bound to different solid supports

The structure of the 19-amino acid peptide epitope, corresponding to the 141-159 sequence of capsid viral protein VP1 of foot-and-mouth disease virus (FMDV), bound to three different resins, namely, polystyrene-MBHA, PEGA, and POEPOP, has been determined by high-resolution magic angle spinning (HRMAS) NMR spectroscopy. A combination of homonuclear and heteronuclear bidimensional experiments was used for the complete peptide resonance assignment and the qualitative characterization of the peptide folding. The influence of the chemicophysical nature of the different polymers on the secondary structure of the covalently attached FMDV peptide was studied in detail. In the case of polystyrene-MBHA and polyacrylamide-PEGA resins, the analysis of the 2D spectra was hampered by missing signals and extensive overlaps, and only a propensity toward a peptide secondary structure could be derived from the assigned NOE correlations. When the FMDV peptide was linked to the polyoxyethylene-based POEPOP resin, it was found to adopt in dimethylformamide a helical conformation encompassing the C-terminal domain from residues 152 to 159. This conformation is very close to that of the free peptide previously analyzed in 2,2,2-trifluoroethanol. Our study clearly demonstrates that a regular helical structure can be adopted by a resin-bound bioactive peptide. Moreover, a change in the folding was observed when the same peptide-POEPOP conjugate was swollen in aqueous solution, displaying the same conformational features as the free peptide in water. The possibility of studying solid-supported ordered secondary structures by the HRMAS NMR technique in a wide range of solvents can be extended either to other biologically relevant peptides and proteins or to new synthetic oligomers.

NMR spectroscopy in drug discovery: tools for combinatorial chemistry, natural products, and metabolism research

NMR spectroscopy has enjoyed many advances recently, and the pace of development shows no signs of slowing. This article focuses on advances that have affected solution-state NMR. These advances fall into three general categories: new experimental techniques (new pulse sequence tools), improved hardware and more powerful software. These advances are allowing NMR to help solve important problems in the field of drug discovery. Their impact is widespread. NMR spectroscopy is now being used to determine protein structures, to monitor ligand-receptor binding, to study diffusion, to analyze mixtures using LC-NMR, to analyze solid-phase synthesis resins and to determine the structures of organic small molecules. NMR spectroscopy can provide both qualitative and quantitative information, and can be used in both routine analytical applications and demanding research applications. The applications described here can benefit numerous disciplines in drug discovery, including natural products research, synthetic medicinal chemistry, metabolism studies, drug production, quality control, rational drug design and combinatorial chemistry.

Surfactant Mediated Cationic and Anionic Suspension Polymerization of PEG-Based Resins in Silicon Oil: Beaded SPOCC 1500 and POEPOP 1500

A novel surfactant has been synthesized for use in cationic and anionic ring-opening suspension polymerization of PEG-based macromonomers in silicon oil. A polymer of acrylate esters containing pentamethyldisiloxane and PEG was prepared by radical polymerization. The surfactant can stabilize an emulsion of PEG-based macromonomers, initiator, and solvent in silicon oil such that polymer beads are obtained by ring-opening polymerization, initiated either by a Lewis acid (cationic ring opening) or potassium tert-butoxide (anionic ring opening). The average bead size could be controlled by varying the stirring rate and the amount of surfactant and solvent. The surfactant does not interfere with the polymerization and can be removed together with residual silicon oil by a simple washing procedure.

Quantitative Determination of Resin Loading in Solid-Phase Organic Synthesis Using (13)C MAS NMR

The use of quantitative carbon nuclear magnetic resonance spectroscopy ((13)C NMR) for the determination of resin loadings has been investigated. Magic angle spinning (MAS) NMR spectra have been obtained for solvent-swollen resins on a conventional 7 mm CP/MAS probe using the two pulse phase modulation (TPPM) proton decoupling sequence. Loadings of resin-bound organic compounds were evaluated via addition of tetrakis(trimethylsilyl)silane as reference or using the carbon resonances of the polymeric resin material as an internal standard. Results for several functionalized Wang and trityl resins are consistent with those obtained using well-established analytical methods. The (13)C NMR method has interesting applications in the field of solid-phase organic synthesis (SPOS), since no functional group acting as a support for the attachment of a quantifiable chromophore must be available in the material of interest.

Adiabatic TOCSY MAS in liquids

The effect of magic angle spinning (MAS) of liquids upon the performance of various isotropic mixing sequences is investigated. Although the mathematical formalism for isotropic mixing under MAS conditions is similar for both liquids and solids, the mechanism through which the coherence transfer is disturbed is different. In liquids, the use of sample spinning in the presence of both RF and magnetic-field inhomogeneities introduces a modulation of the effective field, which compromises the performance of the conventional mixing sequences. This effect is further amplified by supercycles, which normally improve the performance of the mixing and decoupling experiments. It is demonstrated that adiabatic mixing sequences are less susceptible to such modulations and perform considerably better in TOCSY MAS experiments. The best performance of TOCSY MAS is observed under the rotational resonance condition when the sample appears static in the nutation reference frame.

The Chemical Shift Index method applied to resin-bound peptides

The Chemical Shift Index (CSI) method proposed by Wishart et al. [Biochemistry (1992) 31, 1647-1651] to evaluate the secondary structure of peptides in aqueous solution uses as its reference the chemical shift values of each of the 20 natural amino acids (X) in a typical nonstructured sequence GGXAGG (17-20). In order to apply the CSI method to protected resin-bound peptides, we established a new database of chemical shift values for the same GGXAGG sequences in their protected form and anchored to a polystyrene resin swollen in DMF-d7. The predictive value of this new reference set in the CSI protocol was tested on different resin-bound peptides that were previously characterized by a full NOE analysis.

Development of an easy-to-use mass spectrometric technique to monitor solid-phase reactions on polystyrene supports

By using mass spectrometry as an analytical tool to characterise substituted, cross-linked polystyrene resins, it is possible to directly monitor the progress of the solid-phase reactions performed on these resins without prior cleavage of the resin-bound molecules. Therefore, this is a true on-resin analytical method. The mass-to-charge ratios observed in the mass spectra are readily assigned to fragments of the polymer that include the chemically bound substituents. This is the first time that the formation and breaking of bonds have been directly observed on the polymeric support. Furthermore, the relative intensities of the signals in the mass spectra provide a measure of the completeness of the reaction. Because these measurements are rapidly performed without further chemical transformations or cleavage procedures, and because only minimal amounts of material are needed, this technique could become the solid-phase equivalent of thin-layer chromatography used in classical liquid-phase chemistry.

Preparation of fluorinated linkers: use of 19F NMR spectroscopy to establish conditions for solid-phase synthesis of pilicide libraries

Three fluorinated linkers which are analogues of linkers commonly used in solid-phase peptide synthesis have been prepared. One of the linkers was used in combination with gel-phase 19F NMR spectroscopy to develop conditions for solid-phase synthesis of two libraries of pilicides, i.e. compounds designed to inhibit assembly of adhesive pili in uropathogenic Escherichia coli. Attachment to and cleavage from the linker could be monitored based on the chemical shift of the fluorine atom of the linker. In addition, use of the linker as internal standard allowed quantification and optimization of reactions occurring further away from the linker when fluorinated building blocks were employed. Importantly, high-quality 19F NMR spectra were obtained for compounds linked to a TentaGel resin in a standard NMR tube using an ordinary NMR instrument.

Multistep synthesis of 2,5-diketopiperazines on different solid supports monitored by high resolution magic angle spinning NMR spectroscopy

The solid-phase synthesis of 2,5-diketopiperazines containing the trans-4-hydroxy-L-proline amino acid residue (Hyp) was performed on Ellman polystyrene, polyoxyethylene-polyoxypropylene (POEPOP), polystyrene-polyoxyethylene NovaSyn, and Wang resins, respectively. The reaction pathway allowed the introduction of different functional groups around the bicyclic scaffold in a combinatorial approach, and it generated mixtures of isomers. A detailed characterization of the single reaction steps by high resolution magic angle spinning (HRMAS) NMR spectroscopy was performed. The NMR spectral resolution of the resin-bound intermediates and final products was greatly influenced by the polymer matrix. The POEPOP resin permitted to obtain HRMAS NMR spectra with a resolution comparable with that of the spectra of the molecules in solution. Moreover, configurational and conformational isomers formed during the solid-phase reaction steps could be detected and easily assigned. Therefore, the combination of the HRMAS NMR technique with the use of nonaromatic resins may become an extremely powerful tool in solid-phase organic synthesis. This approach will allow the monitoring of multistep reactions and the conception of on-bead structural studies either on small molecules or on natural and/or synthetic oligomers.

Combined application of analytical techniques for the characterization of polymer supported species

The combined application of a diverse range of analytical techniques is described for the complete analysis of polymer supported molecules. These techniques permit complete description of the FTIR, (1)H NMR, and (13)C NMR spectra. The comparison of supported bicyclo[2.2.2]octane derivatives with analogous species prepared using polymer supported reagents is made.

Combinatorial chemistry as a tool for drug discovery

The question 'will combinatorial chemistry deliver real medicines' has been posed [96]. First it is important to realise that the chemical part of the drug discovery process cannot stand alone; the integration of synthesis and biological assays is fundamental to the combinatorial approach. The results presented in Tables 3.1 to 3.8 suggest that so far smaller directed combinatorial libraries have obtained equivalent results to those obtained previously from traditional medicinal chemistry analogue programs. Unfortunately, because of the long time it takes to develop pharmaceutical drugs there are no examples yet of marketed drugs discovered by combinatorial methods. There are interesting examples where active leads have been discovered from the screening of the same library against multiple targets (e.g. libraries 13, 39, 43, 66, 71 and 76). It is now possible to handle much larger libraries of non-oligomeric structures and the chemistry required for such applications is becoming available. Whether combinatorial approaches can also be adapted to deal with all the other requirements of a successful pharmaceutical (lack of toxicity, bioavailability etc.) is open to question but there are already examples such as cassette dosing [235-237]. However we can still be optimistic about the possibility of larger libraries producing avenues of investigation for the medicinal chemist to develop into real drugs. Combinatorial chemistry is an important tool for the medicinal chemist.

Physical properties of poly(ethylene glycol) (PEG)-based resins for combinatorial solid phase organic chemistry: a comparison of PEG-cross-linked and PEG-grafted resins

Three series of poly(ethylene glycol) (PEG)-based polymers were synthesized and characterized with respect to their physical properties. Polyoxyethylene-polyoxypropylene (POEPOP), polyoxyethylene-polyoxetane (SPOCC), and polyoxyethylene-polystyrene (POEPS-3) were synthesized respectively by anion polymerization, cation polymerization, and radical polymerization. Both bulk and suspension modes were used to synthesize the polymers from derivatized PEG monomers (PEG 400, PEG 900, and PEG 1500). The three supports were compared with two commercially available PEG-grafted supports (TentaGel S OH, ArgoGel-OH) and two polystyrene supports (aminomethylated polystyrene [PS-NH2] and macroporous aminomethylated polystyrene [PLAMS]) with respect to their swelling properties, loading, NMR spectral quality, as well as solvent and reagent accessibility. Loadings of 0.3-0.7 mmol/g were obtained for the PEG-based resins. Swelling of the PEG-based resins was determined to be higher than that of the PEG-grafted resins and polystyrene supports. The PEG-based resins gave better resolved high-resolution NMR spectra than the PEG-grafted resins when examined by magic angle spinning nanoprobe (MAS) NMR spectroscopy. Moreover, fluorescence quenching of polymer bound 2-amino-benzoate by protonation with p-toluenesulfonic acid showed moderate to fast diffusion through the polymer depending on the solvent and the polymer matrix.

Direct-injection NMR (DI-NMR): a flow NMR technique for the analysis of combinatorial chemistry libraries

A new tool for analyzing compound libraries by NMR has been developed. Aliquots of solution-state samples (between 120 and 350 microL) are directly injected, using a standard liquids handler, into an NMR (LC-NMR) flow probe. Automated NMR software tracks--and suppresses--intense signals arising from the nondeuterated solvents used (if any) and acquires high-sensitivity one-dimensional 1H NMR spectra. An 88-member combinatorial library, dissolved in DMSO and stored in a 96-well microtiter plate, has been analyzed a number of ways using this technique. This nondestructive technique, which we call direct-injection NMR (DI-NMR) and which is embodied in our versatile automated sample changer (VAST) hardware, has proven to be both routine and robust. Our success in automatically acquiring the NMR data for entire plates of library compounds (within 4-8 h) has caused us to develop new ways to display and analyze the resulting NMR data, as will be shown here.

Characterization of polyalkylvinyl ether phases by solid-state and suspended-state nuclear magnetic resonance investigations

Pure organic polyalkvlvinyl ether phases were synthesized by suspension polymerization using different ratios and compositions of n-butylvinyl ether (C4VE) and n-octadecylvinyl ether (C18VE) with triethylene glycol divinyl ether or divinylbenzene as crosslinkers, respectively. These phases were investigated by means of solid-state 13C cross-polarization magic angle spinning nuclear magnetic resonance (NMR) spectroscopy and 1H high-resolution magic angle spinning (HR MAS) NMR spectroscopy in suspended-state. A comparison of these two methods showed the substantial advantages of 1H HR MAS NMR measurements. Structure elucidation was achieved using a 2D H,H-COSY NMR experiment performed under MAS conditions enabling full peak assignment of the 1H NMR spectra of these phases. The dynamic behavior of the polyalkylvinyl ether phases was determined by employing temperature-dependent measurements of spin-lattice relaxation times (T1) as well as accumulation of a 2D wide line separation NMR spectrum.

Magnetic field gradients in solid state magic angle spinning NMR

Magnetic field gradients have proven useful in NMR for coherence pathway selection, diffusion studies, and imaging. Recently they have been combined with magic angle spinning to permit high-resolution measurements of semi-solids, where magic angle spinning averages any residual dipolar couplings and local variations in the bulk magnetic susceptibility. Here we show the first examples of coherence pathway selection by gradients in dipolar coupled solids. When the gradient evolution competes with dipolar evolution the experiment design must take into account both the strength of the dipolar couplings and the means to refocus it. Examples of both homonuclear and heteronuclear experiments are shown in which gradients have been used to eliminate the need for phase cycling.

NMR probe for the simultaneous acquisition of multiple samples

A dual channel probe for the simultaneous acquisition of NMR data from multiple samples has been developed. This multiplex probe consists of two noninteracting sample coils that are each capable of detecting NMR signals at the same resonant frequency with good sensitivity and resolution. 13C free induction decays for the two samples, methanol (13C, 99%) and carbon tetrachloride (13C, 99%), were acquired simultaneously at 75.44 MHz using a single transmitter pulse and separate NMR receivers. S/N measurements are comparable to those observed using single coils. No evidence of cross talk is evident in the spectra even after considerable signal averaging. The probe demonstrates the feasibility of significant parallelism in NMR, which will be of interest in situations where high throughput analysis is desired.